1. Field of the Invention
The present invention relates to an apparatus for improved control of the blur spot drift in an acousto-optic device due to the thermal response of a solid state laser source.
2. Description of Prior Art
The term acousto-optics (A/O) refers to an interaction of light and sound. Typically an RF input signal is first transformed into an acoustic wave in a suitable crystal material, such as lithium niobate. Variation in index of refraction due to the propagation of the acoustic wave within the crystal can be then used to deflect a beam of light, usually monochromatic. This process is the equivalent of the better known Bragg diffraction of X-rays from the planes of a crystal lattice; for this reason the device is called a Bragg deflector or Bragg cell as well as an acousto-optic deflector or modulator. The angular deflection of the optical beam is proportional to the frequency of the original RF input signal. As the process is linear, multiple simultaneous RF input signals yield multiple simultaneous beam deflections corresponding to the distinct input frequencies with the intensity of the individual deflected beams being proportional to the power of the original RF input signals.
Acousto-optics have been used for a variety of applications where light must be modulated or deflected. An important application is the use of acousto-optics for wideband receiving systems. The acousto-optic phenomenon occurs over a substantial bandwidth, 1 GHz with existing devices, so that the frequency content of an unknown signal environment can be resolved by measuring the angle of deflection corresponding to each signal in the environment. Thus, the entire signal environment may be viewed simultaneously by a device that acts like a channelized receiver.
Due to its inherent temperature stability, a helium neon (HeNe) laser has been the laser of choice for use in an acousto-optic device or system. In recent years, the small size and weight, lower power consumption, and high efficiency of a gallium arsenide (GaAs) based solid state laser has made it an attractive alternative to an HeNe laser. This is particularly true for airborne A/O receiver applications, and other contexts where these advantages offer improved functional performance.
The major disadvantage associated with the GaAs class of solid state laser is strong thermal dependence, because lasing action is related to band gap width which is a function of temperature. Thermal variation of both optical power and wavelength can be observed for devices of this type. Optical power variation can be monitored and controlled, but wavelength variation is a more difficult problem.
A need continues to exist for controlling either laser wavelength or for forcing an acousto-optic device or system to have a small sensitivity to laser wavelength variation.
The only prior art approach which successfully addresses this problem is to imbed the entire optical package in a refrigerator/oven, thus allowing regulation of temperature and minimizing temperature variation.